Numerous methods of end point detection for etch processes are known. Many of them are based on laser interferometry or optical emission spectroscopy, which monitor the interference of light reflected from thin films as they are being etched, or the optical emission respectively. They use similar techniques, but different light sources. Laser interferometry uses a laser beam, such as a helium-neon laser, as a light source, whereas optical emission uses light from plasma emissions in a plasma reaction chamber as the light source.
A thin film interferometry approach is illustrated in FIG. 1. The incoming laser beam 11 is transparent to a thin film 12 which is to be etched. The light beam is reflected from the top surface of the thin film 12 and from the surface of the substrate 13. These reflected beams 14 and 15 respectively interfere. The thickness of the thin layer 12 is related to the wavelength (.lambda.) of the laser beam 11 in accordance with the equation EQU 2d=N(.lambda./n) (1)
wherein N is an integral and n is the refractive index of the thin film 12. For integral values of N (1, 2, 3 etc) the interference is constructive and the reflected intensity is a maximum; whereas for half integral values (N=1/2, 3/2, 5/2 etc) the reflected light interferes destructively and the intensity is at a minimum. During etching, a characteristic sinusoidal optical interference pattern of repetitive maxima and minima is monitored during etching. When the layer 12 is etched away, this pattern terminates, giving an end point signal.
FIG. 2 illustrates a typical pattern of the intensity of the reflected beam over time. The equation describing the intensity of a reflected beam modulated by interference due to a change in film thickness as it is being etched is ##EQU1## wherein I is intensity, n is the index of refraction of the film being etched, d.sub.t is the film thickness changing over time and .lambda..sub.0 is the center of the wavelength of the incoming light beam in air or vacuum.
The etch rate is related to the change in time between the occurrence of two maxima, as shown in FIG. 3, and can be determined in accordance with equation 3: ##EQU2## The end point is reached when the intensity reaches a maximum and then remains unchanged, indicated at point EP in FIG. 3. Thus monitoring reflected light enables one to determine the etch rate and etch end point of a film on a substrate.
However, these known end point detection systems are useful for dry etching processes, e.g., plasma or reactive ion etching. If they are used for wet etch processes, they are required to ensure a constant thickness of the wet etchant in order to suppress unwanted interference effects.
An etch apparatus is available that accommodates a substrate having one or more thin films thereon to be wet etched to remove one or more of the films from one side only (frontside or backside) using known etch solutions. Such apparatus is employed to remove thin films from the one side of a substrate such as a silicon wafer, particularly from the back side, but also from the front side. A flow of wet etchant is supplied to that side of the wafer which faces up. By spinning the wafer, a film of wet etchant is formed whose thickness varies irregularly. Although the etch rate of particular etchants for particular films may be generally known, state-of-the-art semiconductor processing requires automated, in situ processing and very accurate end point detection also for this one side wet etch apparatus. Such a method does not presently exist for one side wet etch processes. Thus it would be highly desirable to be able to monitor etch processing using a one side etch apparatus and to determine the true end point.